Survey for Parasitoids of Swede Midge, an Important Pest of - Ontario Soil and Crop ...
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Crop Advances: Field Crop Reports
Survey for Parasitoids of Swede Midge, an Important Pest of
Canola
Funding by OMAFRA/UG – Interim Report
Purpose:
Swede midge continues to pose a significant threat to canola production in Ontario.
Based on farmers’ experience with the pest and the difficulty of controlling swede midge
through insecticide applications alone, it is clear that multiple tactics are required for
successful swede midge management. Beneficial insects that parasitize swede midge
could be valuable in controlling the pest. Knowledge of the presence and activity of
parasitoids will lead to refined integrated pest management strategies.
Figure 1. Adult Swede Midge Caught In A Pheromone Sticky Trap.
A survey of oilseed rape in Europe in 2008-2011 identified some possible beneficial
insects including the parasitic wasp Synopeas myles (Figure 2). Surveys in Ontario, led
by the research team of Dr. Rebecca Hallett, University of Guelph, have shown that this
insect is present in Ontario as well. The purpose of this study was to identify the
presence of S. myles in Ontario canola, and to determine to what degree S. myles is
able to parasitize and control swede midge.
Methods:
Sampling for S. myles across the canola growing regions of Ontario was conducted in
2017 and 2018. In 2017 there were 30 sites where samples were collected once or twice
during the flowering stages of canola in June and July. In 2018 samples were collected
at 26 sites. Sample locations were within the following counties or districts: Cochrane,
Temiskaming, West Nipissing, Renfrew, Peterborough, Durham, Dufferin, Wellington
and Bruce.
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Crop Advances: Field Crop Reports
Figure 2. Synopeas myles, A Non-Specific Parasitoid of Swede Midge. Photo
Credit: Dr. Elijah Talamas,
Swede midge larvae are found on fast growing tissue of brassica plants, such as within
flower bud clusters. Canola plants were sampled in the flowering stages so that there
were multiple growing points on each plant. At each site, 20 plants that showed signs of
swede midge damage were cut at the soil surface and individually bagged (Figure 3).
Damaged plants are typically easy to find at field edges. Samples were kept cool until
they were delivered to the University of Guelph.
Figure 3. Example of a plant identified to have swede midge damage and collected
for the survey. Plants that showed damage at multiple growing points were
preferred for sampling. Cutting open this bud cluster would reveal the tiny swede
midge larvae.
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Crop Advances: Field Crop Reports
S. myles females lay an egg inside the bodies of developing swede midge larvae, and
the larvae continue to develop with a wasp growing inside. Eventually the parasitized
swede midge larvae drop to the ground to form a cocoon in the soil as they normally
would. However, an S. myles wasp will emerge rather than an adult swede midge.
The growing points were cut from each canola plant and placed in soil in a closed
container (Figure 4). The number of adult swede midge and S. myles that emerged from
the soil were counted, and percent parasitism was determined.
Figure 4. Growing points of individual sampled canola plant were placed in a
closed container with soil and observed for emerging insects. Photo credit:
Charles-Étienne Ferland
In 2016 through 2018, sites near Shelburne and Meaford were sampled weekly using
similar methods to determine how long S. myles is present during the growing season.
Parasitic wasps collected from containers were sent to experts in Copenhagen and
Saskatoon for visual and molecular identification.
Results:
S. myles is not native to Canada. It is also not a specific parasitoid of swede midge; it
will also parasitize other types of midges. The wasp lays its eggs within swede midge
larvae, meaning the pest is already present on canola plants and some crop damage
may have already occurred. These factors mean that S. myles will not be a silver bullet
in controlling swede midge and preventing crop damage, but there is potential for the
wasp to provide some control of swede midge populations. For effective control of
swede midge, multiple tactics which all help to reduce populations and the potential for
population growth are needed.
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Crop Advances: Field Crop Reports
Although the parasitoid was not found in the samples from every farm field, it was found
in at least one field of each county or district sampled. The identification of S. myles in
important canola growing regions of Temiskaming and West Nipissing is promising in
that the non-native beneficial is being found in more northern regions under fairly limited
sampling procedures. It is likely that S. myles is located at more sites than indicated by
the results, since sampling was limited in terms of both the number of times a site was
sampled and the number of plants collected during the season.
Table 1. Abundance of S. Myles Found In Ontario Canola Fields, and Dates of
Earliest and Latest Detection in 2016- 2018.
Assessment 2016 2017 2018
No. of Sites Sampled 4 30 26
No. of Sites Positive for 4 13 19
Parasitic Wasp
Sampling Date of Earliest June 23 June 28 July 16
Wasp Detection
Sampling Date of Latest Aug 19 Sept 11 Aug 29
Wasp Detection
Total No. of Wasps 6040 260 1077
One site had 5060, One site had 150, One site had 470,
others had 100 to others had 1 to 35. others had 5 to 60.
700.
There appears to be about 11 weeks of the season during which S. myles can be found.
Swede midge emerge from the soil at the end of May or beginning of June. The wasp is
typically first detected 1 to 2 weeks after swede midge first emerge. Rates of parasitism
averaged 5.5% in Ontario, compared to 2.9% in the European survey. The maximum
level of parasitism by S. myles observed at an Ontario location was 30%. Relatively high
levels of parasitism were observed in 2016, and in the week of July 7th all 4 sample sites
had an average of 27.9% parasitism of swede midge. However, in 2017 abundance of S.
myles was much lower, and the highest parasitism rate reached only 4.1%. A second
species of beneficial wasp, from the Platygastridae family, was identified in some
samples. Environmental and individual field differences across years likely affected both
the swede midge and beneficial insect populations.
To date, the observed levels of parasitism are much lower than those required to control
swede midge sufficiently to prevent yield loss on their own. Researchers have reported
that parasitoids that provide rates of parasitism under 30-40% may not have a large
impact on target pests. Anecdotally, famers and agronomists in the Shelburne area have
noted in recent years that swede midge populations climb rapidly at first then tend to
level off. They question whether this is related to the abundance of S. myles found in
that area.
4Crop Advances: Field Crop Reports
Multiple approaches will always be required to control this difficult pest in canola.
However, continued research on the environmental factors affecting parasitoid presence
and abundance, and the impact of crop management practices on the beneficial, could
lead to improved stewardship of these helpful insects and higher rates of parasitism.
Next Steps:
Dr. Hallett’s lab will continue surveying for S. myles, and hope to gain a better
understanding of the parasitoid’s population within swede midge’s invasive range. A
better understanding of this parasitoid may lead to opportunities to promote its activity.
Through the establishment of a captive population of S. myles, research will continue on
the environmental factors affecting its survival and rates of parasitism. Insecticides that
are used for controlling swede midge on farms will be evaluated for their relative activity
on S. myles in an effort to reduce negative impacts on this beneficial insect.
Acknowledgements:
This report was co-authored with Dr. Rebecca Hallett and Charles-Étienne Ferland (MSc
Candidate) from the University of Guelph, and Meghan Moran, OMAFRA Canola &
Edible Bean Specialist.
Many canola growers and agronomists from across the province participated in the
survey efforts and their support is greatly appreciated.
Funding provided by the OMAFRA/UG Partnership.
Project Contacts:
Meghan Moran
Canola & Edible Bean Specialist, OMAFRA
Meghan.moran@ontario.ca
519-546-1725
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